From: Gabriel Shahrouzi gshahrouzi@gmail.com
[ Upstream commit 0ba3a4ab76fd3367b9cb680cad70182c896c795c ]
Move the get_ctx(child_ctx) call and the child_event->ctx assignment to occur immediately after the child event is allocated. Ensure that child_event->ctx is non-NULL before any subsequent error path within inherit_event calls free_event(), satisfying the assumptions of the cleanup code.
Details:
There's no clear Fixes tag, because this bug is a side-effect of multiple interacting commits over time (up to 15 years old), not a single regression.
The code initially incremented refcount then assigned context immediately after the child_event was created. Later, an early validity check for child_event was added before the refcount/assignment. Even later, a WARN_ON_ONCE() cleanup check was added, assuming event->ctx is valid if the pmu_ctx is valid. The problem is that the WARN_ON_ONCE() could trigger after the initial check passed but before child_event->ctx was assigned, violating its precondition. The solution is to assign child_event->ctx right after its initial validation. This ensures the context exists for any subsequent checks or cleanup routines, resolving the WARN_ON_ONCE().
To resolve it, defer the refcount update and child_event->ctx assignment directly after child_event->pmu_ctx is set but before checking if the parent event is orphaned. The cleanup routine depends on event->pmu_ctx being non-NULL before it verifies event->ctx is non-NULL. This also maintains the author's original intent of passing in child_ctx to find_get_pmu_context before its refcount/assignment.
[ mingo: Expanded the changelog from another email by Gabriel Shahrouzi. ]
Reported-by: syzbot+ff3aa851d46ab82953a3@syzkaller.appspotmail.com Signed-off-by: Gabriel Shahrouzi gshahrouzi@gmail.com Signed-off-by: Ingo Molnar mingo@kernel.org Cc: Peter Zijlstra peterz@infradead.org Cc: Ravi Bangoria ravi.bangoria@amd.com Cc: Kan Liang kan.liang@linux.intel.com Cc: Oleg Nesterov oleg@redhat.com Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Link: https://lore.kernel.org/r/20250405203036.582721-1-gshahrouzi@gmail.com Closes: https://syzkaller.appspot.com/bug?extid=ff3aa851d46ab82953a3 Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/events/core.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c index f6cf17929bb98..6503ff28a782c 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -13729,6 +13729,9 @@ inherit_event(struct perf_event *parent_event, if (IS_ERR(child_event)) return child_event;
+ get_ctx(child_ctx); + child_event->ctx = child_ctx; + pmu_ctx = find_get_pmu_context(child_event->pmu, child_ctx, child_event); if (IS_ERR(pmu_ctx)) { free_event(child_event); @@ -13751,8 +13754,6 @@ inherit_event(struct perf_event *parent_event, return NULL; }
- get_ctx(child_ctx); - /* * Make the child state follow the state of the parent event, * not its attr.disabled bit. We hold the parent's mutex, @@ -13773,7 +13774,6 @@ inherit_event(struct perf_event *parent_event, local64_set(&hwc->period_left, sample_period); }
- child_event->ctx = child_ctx; child_event->overflow_handler = parent_event->overflow_handler; child_event->overflow_handler_context = parent_event->overflow_handler_context;